1. Field of the Invention
The present invention relates to a developing apparatus, preferably employed for an image processing apparatus that uses an electrophotographic system, and to a detachable processing cartridge, which is also employed for the image processing apparatus.
This image processing apparatus in question can be, for example, an electrophotographic printer that uses an electrophotographic system, such as an electrophotographic copier, an LED printer or a laser beam printer, or an image processing apparatus that uses an electrostatic recording system, such as an electrophotographic facsimile machine. The processing cartridge is provided by integrally constituting an image bearing member, such as an electrophotographic photosensitive member, and, at the least, one either of electrifying means, developing means or cleaning means and an image bearing member, and is detachable from the main body of the image processing apparatus.
2. Related Background Art
In an electrophotographic image processing apparatus, such as a copier or a laser beam printer, light is emitted by exposure means in accordance with image data to an electrophotographic photosensitive member (hereinafter referred to simply as a photosensitive member), which is an image bearing member charged by electrifying means, and a latent image is formed thereon. A developing apparatus supplies to the latent image a developer (toner), which is a recording material, and visualizes the latent image to obtain a developed image (toner image). Then, the toner image is transferred from the photosensitive member to a recording sheet, a recording medium, so as to record the image thereon.
A developer container (a toner container), which is a toner storage unit, is connected to the developing portion of the developing apparatus, and toner is consumed as images are formed. For many image processing apparatuses, the developing apparatus, which includes the toner container as the developing means, the photosensitive member, which serves as the image bearing body, and the electrifying means, which electrifies the surface of the photosensitive member so that a latent image can be formed thereon, are integrally formed and function as a processing cartridge. When the supply of toner is exhausted in a processing cartridge, a user need only exchange the processing cartridge to again perform the image forming processing.
Practical use has been made of various electrophotographic developing methods. And since a developing apparatus having the simplest structure has the fewest troubles and the longest service life and is the most easily maintained, a one-component developing method using a magnetic toner has been widely employed.
Recently, since higher speed and longer service life are capabilities requested for a processing cartridge because the speed of a laser beam printer is thereby increased, an increase in the amount of toner loaded in a processing cartridge is sought, and thus, the capacity of a toner container must also be increased.
To increase the speed and the capacity of a processing cartridge, a higher processing property is requested for toner, and since the following (A) low-temperature fixing properties and (B) electrification stability properties are especially important, various proposals have been made.
An explanation will now be given for (A) a low-temperature fixing property.
For a fast developing system, toner for which polyester is the main component of a bonding resin is generally employed in order to prevent a fixing failure during the fixing processing performed to fix toner to a recording medium using heat and pressure.
Compared with the styrene-acrylic toner that has been widely employed, polyester toner has a lower glass transfer temperature (Tg) and a superior low-temperature fixing property, and is appropriate for a fast fixing system.
However, since polyester is highly hygroscopic and hydrophilic, in an environment at a high temperature and a high humidity, the flowability of the toner is deteriorated due to the hygroscopicity of the polyester resin, and various problems occur.
As for polyester toner, a technique is disclosed in Japanese Patent Application Laid-Open No. 2000-147832, according to which the ratio of the content of a specific molecular component of the polyester resin is changed in order to improve the low-temperature fixing property and a high-temperature resistance offset property. Further, another technique is disclosed in Japanese Patent Application Laid-Open No. 6-242630, according to which the surface property and the flowability of toner are improved by using hydrophobic silica as an outward additive.
However, these two proposals are not related to the techniques employed for improving the hydrophobic property of polyester toner, and still more improvement is required for the flowability of toner in a high temperature and high humidity environment.
An explanation will now be given for (B) electrification stability.
Electrification stability is important for a developing system in which a large amount of toner is stored, i.e., the toner electrification amount must be quickly increased, the environment stability must be superior, and stable electrification of the toner must be performed for an extended period of time.
Magnetic iron oxide is contained in toner to provide magnetism for the toner, and a state wherein magnetic iron oxide is dispersed across the surface of the toner particles greatly influences the electrification property of the toner.
As less magnetic iron oxide is exposed at the surface of the toner particles, i.e., as more of the surface of the toner particles is covered with the bonding resin component, the electrification property of the toner is improved, and a superior development function is obtained. On the other hand, this may cause over-electrification in a low temperature and low humidity environment.
On the other hand, as more magnetic iron oxide is exposed on the surface of the toner particle, i.e, as less of the surface of the toner particle is covered with the bonding resin component, the magnetic iron oxide on the surface of the toner particle serves as an electrification leakage site. Therefore, especially in a high temperature and high humidity environment, the toner is not satisfactorily electrified and deterioration of the developing function occurs, which causes an image failure.
More natural is for the electrification property of the toner to be improved by using an outward additive, such as inorganic particles or a metal oxide material. However, for a developing system for which a long service life is desired, a change in the electrification property, which occurs due to a separation or a parting of the outward additive after it has been in use for a long time, must be avoided. Therefore, it is important that the toner electrification amount be controlled by using a so-called toner parent body, such as a bonding resin or magnetic iron oxide, rather than by using an outward additive. That is, it is important that an appropriate amount of the magnetic iron oxide exposed on the surface of the toner particle be maintained.
To obtain the amount of magnetic iron oxide present on the surface of the one-component magnetic toner particles, a method is available for evaluating the hydrophobicity of a specific toner solvent, i.e., for evaluating the wettability of a specific organic solvent.
In the wettability evaluation, the wetting (sedimentation) of toner for a specific organic solvent is measured by permeability, and the wettability (the hydrophobicity relative to water) of the specific organic solvent is obtained. That is, when wetting of a toner tends to occur at a low density, it means that its hygroscopic property is high. Generally, a solvent mixture consisting of methanol and water is employed as a specific organic solvent.
The wettability of magnetic toner relative to a methanol-water mixture solvent is greatly influenced by the composition of the surface material of the one-component magnetic toner particles and the existing material state. As less magnetic iron oxide is exposed on the surface of the toner particles, i.e., when more of the surface of the toner particle is covered with the bonding resin component, the wetting of the magnetic toner occurs at a high methanol concentration, i.e., wetting of the toner is not easy. On the other hand, as more magnetic iron oxide is exposed on the surface of the toner particle, i.e, when less of the surface of the toner particle is covered with the bonding resin component, the wetting of magnetic toner occurs at a low methanol concentration, i.e., the wetting of the magnetic toner is easy.
When more magnetic iron oxide is exposed on the surface of the toner particles, the electrification capability is reduced and the wettability is deteriorated, so that in a high temperature and high humidity environment, the flowability of the toner tends to be deteriorated. In other words, in order to prevent the magnetic toner from being over-charged in a low temperature and low humidity environment, and to stabilize the long term electrification of the toner, the magnetic iron oxide on the surface of the toner particles must be exposed and dispersed, to a degree; however, the magnetic iron oxide exposed on the surface of the toner particles deteriorates both the wettability of the toner and the flowability of the toner in a high temperature and high humidity environment.
Especially for a fast developing system that stores a large amount of toner, since from the viewpoint of image fixing at a low temperature, polyester toner must be employed, the flowability of toner in a high temperature and high humidity environment is more remarkably deteriorated due to hygroscopicity (hydrophile), and the following problems occur.
As a first problem, since toner is not uniformly supplied to a developing roller that is a developer carrying member for carrying toner from a toner container to a photosensitive member, an image failure, such as fading, occurs.
As a second problem, since toner that is scraped from the developing roller by a regulating member remains on the reverse side of the developing roller, and since over-charging occurs because of the continuous fiction of the toner against the developing roller, a reduction in image density and an image failure such as fogging occur.
As a third problem, since a toner that cannot be circulated due to poor flowability is maintained on the reverse side of the developing roller, and since the toner is subjected to friction heat generated near the developing roller, the toner is thermally damaged (toner deterioration) and an image failure occurs.
As a fourth problem, since packing for toner having poor flowability tends to occur due to the weight of the toner in a large toner container, and when toner in this state is retained in an environment at a high temperature and a high humidity, blocking in the toner container will occur.
To resolve the first to fourth problems, an improvement in the flowability of toner is proposed that uses a technique disclosed in Japanese Patent Application Laid-Open No. 7-281478 for changing the property of a resin, a technique disclosed in Japanese Patent Application Laid-Open No. 2000-284522 for changing the flowability using wax contained in the toner particles, or a technique disclosed in Japanese Patent Application Laid-Open No. 6-230604 for changing the flowability index of a magnetic member.
Further, while taking the hydrophobicity of toner into account, toner containing high hydrophobic silica as an outward additive is disclosed in Japanese Patent Application Laid-Open No. 2000-310884.
Furthermore, for the structure of a developing apparatus that stores a large amount of toner, a technique is disclosed in Japanese Patent Application Laid-Open No. 2001-201931 according to which a toner container in which toner is stored is defined to provide a plurality of chambers, and the toner is agitated and carried by agitating members provided in the individual chambers.
According to this technique, since the toner weight can be dispersed and the circulation of toner in the toner container is greatly enhanced, toner deterioration due to poor toner circulation does not occur, even when a large amount of toner is stored in a toner container, and preferable images can be obtained when the toner is used for an extended period of time.
However, since the flowability improvement techniques disclosed in Japanese Patent Application Laid-Open Nos. 7-281478, 2000-284522 and 6-230604 respectively provide for the changing of the property of the resin, by changing the flowability of the wax contained in the toner particles, and by changing the flowability index of the magnetic member, the properties of various toner materials can be changed to obtain a desired toner index flowability, while the costs will be increased and a preferable development characteristic may not be obtained.
In addition, according to the method disclosed in Japanese Patent Application No. 2000-310884 for using an outward additive, while the outward additive is highly hydrophobic, it is difficult to say that the original hydrophobicity of the toner will be achieved because the state wherein the outward additive is dispersed across the surface of the toner particles will differ, depending on an outward additive type. Especially for a fast developing system for which a large amount of toner is used, it is important that, when the toner usage term is long, a stable toner electrification amount be maintained without the toner characteristic being changed, and an improvement in the toner characteristic (hydrophobicity), using only the outward additive, will not be satisfactory.
Further, for a fast developing system requested for long-term use, it is not sufficient for a highly hydrophobic toner to be employed, and it goes without saying that optimization of the structure of the developing apparatus must be a requisite.
Furthermore, for the technique disclosed in Japanese Patent Application Laid-Open No. 2001-201931, wherein the toner container for the developing apparatus, in which a large amount of toner can be stored, is defined as including a plurality of chambers, and the toner is agitated and carried by agitating members, provided in the individual chambers, and an increase in the size of the toner container can not be avoided, and since multiple agitating members are required, the size and the costs will also be increased.
There is another method proposed, whereby control means, for controlling the rotational speed of an agitating member, and agitating member driving means are provided, and the rotational speed of the agitating member is changed in accordance with the environment and state of the usage, so that the toner electrification amount is stabilized and circulation of the toner is controlled. According to this method, however, since both the driving means and the control means are required, the size and the cost of the developing apparatus are also increased.